1. Field of the Invention
The present invention relates to a process for forming high quality diamond films on single crystal diamonds, polycrystalline diamond films and diamond particles by vapor-phase synthesis, which are used for semiconductors and optical devices.
2. Description of the Related Art
Diamond has high hardness, high thermal conductivity, and an excellent stability against heat, radiation and chemicals. In recent years, it becomes possible to form by vapor-phase synthesis using a chemical vapor deposition apparatus. The diamond film thus formed are used as coatings for cutting tools, speaker diaphragms, heat sinks for integrated circuits (ICs) and the like. Diamond is an electrically insulating material but becomes a p-type semiconducting material when doped with boron (B). Semiconducting diamond has a large band gap of about 5.4 eV, and withstands a high temperature of several hundred .degree. C. Research activities have been conducted to develop electronic devices such as diodes, transistors and sensors employing p-type semiconducting diamond films.
Generally, insulating diamond films are formed by vapor-phase synthesis using a source gas containing C, H or O, such as CH.sub.4 --H.sub.2 --O.sub.2, CH.sub.4 --H.sub.2 --CO.sub.2, CH.sub.3 OH--H.sub.2, and CO--H.sub.2 mixed gases. Such mixed gases are decomposed by microwave, heat or high frequency electromagnetic energies to deposit a diamond film on a substrate.
B-doped p-type semiconducting diamond films are formed by vapor-phase synthesis by adding B.sub.2 H.sub.6 gas, or a boron-containing compound gas produced by dissolving solid B.sub.2 O.sub.3 in acetone and bubbling it. Alternatively, B-doped p-type semiconducting diamond films may be formed by disposing solid boron (B) or solid B.sub.2 O.sub.3 on or near a substrate during vapor-phase synthesis.
FIG. 1 shows a relationship between the size of diamond particles deposited in the initial stage of vapor-phase synthesis and the deposition time when diamond films are deposited on a substrate made of non-diamond material such as Si or Mo. In FIG. 1, step A is a nucleation stage on the substrate, showing that a certain period of time is necessary for the nucleation. In step B, the nuclei grow to be diamond particles, and the particle size become larger with time. As the synthesis is further continued, the diamond particles are connected to each other to form a continuous diamond film.
In general, when diamond is deposited by vapor-phase synthesis, graphite and amorphous carbon are simultaneously deposited, causing crystal defects in diamond. It is well known that non-diamond components are selectively etched by hydrogen in the source gas. The selective etching effect is more effective by the addition of oxygen, leading to a lower density of crystal defects and better crystallinity of diamond.
FIG. 2 shows a mixing condition of the atomic concentrations of carbon [C], hydrogen [H] and oxygen [O] in the source gas when diamond is deposited on non-diamond substrates. In this figure, region A shows the mixing condition of [C], [H] and [O] in the source gas when non-diamond components are mainly deposited; region B shows the mixing condition of [C], [H] and [O] in the source gas when diamond is mainly deposited; and region C shows the mixing condition of [C], [H] and [O] in the source gas when diamond is rarely grown. The conditions under which diamond is deposited by vapor-phase synthesis do not depend on the decomposition method of the source gas, but are when carbon atom concentration [C], hydrogen atom concentration [H] and oxygen atom concentration [O] in the source gas are present in the range shown by region B of FIG. 2 (P. K. Bachmann, D. Leers and H. Lydtin, Diamond and Related Materials, Vol. 1, p. 1, 1991; hereinafter, referred to as the prior art 1). When diamond is deposited under the mixing condition of the source gas in region B near the boundary with region A, wherein [H] and [O] are smaller with respect to [C], non-diamond components remain in diamond because of the shortage of hydrogen and oxygen. Under the condition of the region A, as described above, non-diamond components are mainly deposited.
When diamond is deposited under the condition of region B near the boundary with region C, wherein [H] and [O] are greater with respect to [C], non-diamond components are reduced to produce diamond with better quality; especially, when [O] is further increased, the quality of diamond is significantly improved. However, oxygen also etches diamond, therefore, as the mixing condition of the source gas comes close to region C, the nucleation density is reduced. In region C, diamond is rarely grown, because the etching rate of diamond by oxygen becomes higher than the growth rate. The boundary between regions B and C is expressed by [C]/([C]+[O])=0.44.
Unexamined Japanese Patent Publication No. HEI 1-301586 (hereinafter, referred to as the prior art 2) discloses that diamond is deposited on silicon substrates in both region B and part of region C of FIG. 2, when the source gas contains only hydrocarbon and oxygen but not hydrogen. However, the prior art 2 does not disclose mixing conditions to form high quality diamond films on diamond substrates.
In order to deposit high quality diamond films by vapor-phase synthesis, single crystal diamond or polycrystalline diamond films must be used for substrates. Moreover, as a mixing condition for the source gas, [O] is required to be increased with respect to [C]. However, when diamond is deposited under the condition of [C]/([C]+[O]).gtoreq.0.44, large amounts of non-diamond components and crystal defects remain in diamond crystals. As a result, the diamond films thus obtained do not meet the quality requirements for active layers and insulating layers of semiconducting diamond devices.